The present invention relates to model-based predictive control technology for boiler control. More particularly the invention relates to the coordination of air and fuel during transients to increase efficiency and minimize the production of NOx.
The classical approach to combustion air control is to use the measurement of oxygen concentration in flue gas for feedback control of the amount of combustion air. This reactive approach does not guarantee exact air-fuel ration during fast transients. While the standard air-fuel interlock provides acceptable steady-state performance, the solution based on conventional controllers may not be fully satisfactory during the transients, e.g. for boilers operating in cycling regimes, particularly if low-NOx burning with reduced excess air is used.
Lang U.S. Pat. No. 5,367,470 is one of many patents describing the method of analyzing combustion for improved performance, in this case focusing on repetitive adjustment of assumed water concentration in the fuel until actual and calculated values for efficiency reach steady state. Okazaki et al. U.S. Pat. No. 5,764,535 uses two-dimensional or three-dimensional cells in a furnace as part of a system employing a gas composition table to simplify the calculation. Carter U.S. Pat. No. 5,794,549 employs a plurality of burners to form a fireball to optimize combustion. Likewise, Khesin U.S. Pat. No. 5,798,946 converts a fluctuational component of a signal to an extreme point
Chappell et al. U.S. Pat. No. 5,520,123 and Donais et al. U.S. Pat. No. 5,626,085 both disclose systems relating to NOx, using oxygen injection into an afterburner and windbox-to-furnace ratios, respectively. Waltz U.S. Pat. No. 5,091,844 and Blumenthal et al. U.S. Pat. No. 5,496,450 both relate to methodology for control relating to sensor feedback. Finally, Stevers et al U.S. Pat. No. 5,501,159 teaches the use of a jacketed vessel with multiple chambers and air flows.
None of the prior art recognizes the, potential for application of model-based predictive control technology for boiler control that will enable tight dynamic coordination of selected controlled variables, particularly the coordination of air and fuel during the transients.
It would be of great advantage in the art if predictive control technology could be developed that would take into account relatively fast dynamics of boilers and rate limits imposed by the plant life-time considerations.
It would be another great advance in the art if a system could be developed that would focus on power and heat generation to use predictive control technology and rate optimal control to have tight dynamic coordination of selected control variables to result in improved boiler efficiency and reduced NOx production.
Other advantages will appear hereinafter.
It has now been discovered that the above and other objects of the present invention may be accomplished in the following manner. Specifically, the present invention employs inferential sensing to estimate the total amount of combustion air for predictive control of air-fuel ratios for pulverized-coal fired boilers and other boiler systems using other fuels. The invention is useful for any fuel burning system, and has been found to be particularly suited for pulverized coal burning boilers.
Using the estimate of the relation between the total air in the boiler rather than just the measured combustion air added to the boiler, the amount of air can be controlled by a predictive controller. The air to fuel ratio is accomplished in fast transients since the system does not have to wait for real-time feedback from analysis of the exhaust gases. The present invention allows the system to use minimum necessary excess air, thus providing low NOx, production and increased efficiency by at least one percent. The invention contemplates the use of what is termed cautious optimization (cautious optimization is related to the uncertainty in CO and NOx), in which the uncertainty of air entering the system from sources other than directly controlled and measured input is inferentially sensed or estimated from the concentration of O2 measured in the flue gasses, which represents all of the air in the boiler.